"What a lovely gentle breeze there is outside today," someone spoke to a friend in hearing distance of me.
Turning to face them I replied, "Well, actually a gentle breeze is between 3 and 5 mph, whereas the way the limbs on that nearby tree are swaying lead me to believe the wind is actually closer to 8 mph which would be a fresh breeze."
A conversation such as this is not unusual when someone says there is a gentle breeze outside and I often think about the exact mph. This of course is thanks to the Beaufort Wind Force Scale. Although originally developed for use on the sea, it was widely used by meteorologists for on land until recently when mph speeds were brought into use.
Sir Francis Beaufort originally created the wind speed measuring system that carries his name, in 1805.
Evolving to its current position was a process in the making. Beaufort was a top administrator in the Royal Navy and this may have lead to his interest in the weather and its effect on the sea. During his career in the Navy, he noticed while the officers regularly made weather observations, but there was no standard or scale that was required to be followed. This lead to subjectivity whereas one's man's "gentle breeze" may be another man's "strong breeze." If he succeeded in one thing, he was able to make a standard to follow.
When first developed, Beaufort intended the classes to reference wind conditions compared to their effects on the sails of a war ship, rather than wind speed numbers. The scale was made standard for the ship log entries starting in the late 1830's. During the 1850's a cup anemometer was developed for measuring the speed of the wind. It worked by counting the number of rotations a colored cup made on the end of a pole held in the wind.
Due to the increase in steam powered travel, in 1906 the descriptions where changed to describe the effect on the sea instead of the sails. All cup anemometer rotations were scaled to a standardized range in 1923, at this point land descriptions were also introduced due to the fact of use on the land. A few decades later the measure was again altered for use by meteorologists.
In 1946 the scale was extended from 12 to 17 classes. But forces 13-17 are only used in special cases such as tropical cyclones, and mainly in China and Taiwan.
The formula for finding the wind speed is the formula V=0.836 B3/2 m/s with V equaling that of the wind speed 10 meters above sea level and B equaling the class of the current wind speed. m/s is to be calculated according to the anemometer used.
however a homemade anemometer requires a different formula. 0.00595 x radius of the anemometer (in inches) x the number of revolutions a minute. It can be fun, informative and simple to make an anemometer.
Beaufort's scale is currently used in the shipping broadcasts in the United Kingdom.
Beaufort Wind Force Scale:
0 is a calm and the speed of the wind is 0 to 0.2 mph
1 is Light Air and the speed of the wind is 0.3 to 1.5 mph
2 is a Light Breeze and the speed of the wind is 1.6 to 3.3 mph
3 is Gentle Breeze and the speed of the wind is 3.4 to 5.4 mph
4 is a Moderate Breeze and the speed of the wind is 5.5 to 7.9 mph
5 is a Fresh Breeze and the speed of the wind is 8.0 to 10.7 mph
6 is a Strong Breeze and the speed of the wind is 10.8 to 13.8 mph
7 is a Near Gale or Moderate Gale and the speed of the wind is 13.9 to 17.1 mph
8 is a Fresh Gale and the speed of the wind is 17.2 to 20.7 mph
9 is a Strong Gale and the speed of the wind is 20.8 to 24.4 mph
10 is a Whole Gale or Storm and the speed of the wind is 24.5 to 28.4 mph
11 is a Violent Storm and the speed of the wind is 28.5 to 32.6 mph
12 is a Hurricane and the speed of the wind is 32.7 to 40.8 mph
So the next time someone says there is a light breeze outside, ask them if they are a Beaufort Scale expert, because now you are.